Preparation technology of acidic silica sol
1. 1. ion exchange method
This method is the most studied and mature preparation technology at present. This method takes sodium silicate as raw material, which can usually be divided into three steps: preparation of active silicic acid, preparation of alkaline silica sol and cation exchange. Usually, the preparation process is as follows: dilute the commercially available sodium silicate and exchange it with cation exchange resin to obtain active silicic acid; Treating silicic acid with alkaline solution to alkalinity; Heating and concentrating alkaline silicic acid solution, and concentrating to obtain alkaline silica sol; Finally, the alkaline silica sol is cation-exchanged with cationic resin, and an appropriate amount of acid is added for adjustment to obtain acidic silica sol with corresponding acid value.
As early as 194 1, Bird, an American, mentioned in his patent invention that acidic silica sol was prepared by ion exchange method, that is, the alkali metal in sodium silicate was exchanged with hydrogen through a hydrogen cation exchange column, and the product was high-purity acidic silica sol with a pH value of 2.0 ~ 4.0. Since then, Albrecht and William L have improved the preparation process of Bird's acidic silica sol, and proposed to use mixed resin bed to produce more suitable acidic silica sol.
In 1980s, most silica sol manufacturers followed the ion exchange method to prepare acidic silica sol. For example, China Meihua Daily Chemical Plant in Hubei Province began to develop acidic silica sol in July 1985. They prepared acidic silica sol from self-made alkaline silica sol by ion exchange method. The specific process is as follows: after diluting and filtering the required alkaline silica sol, hydrogen cation exchange resin is added under stirring. When the pH value reaches 2 ~ 3, stop adding the resin and let it exchange completely. The content of silica in the acidic silica sol prepared by the above method is more than 10%, the particle size is 10 ~ 20 nm, the pH is 2 ~ 3, and the stable period is 3 ~ 6 months.
Xu Nianqiang and others aged the prepared active silicic acid for 24 ~ 48h, then made alkaline silica sol, and then made acidic silica sol with strong acidic cationic resin. They analyzed the effects of pH value, silica particle size and electrolyte salt concentration on the stability of acidic silica sol, and emphasized that to prepare acidic silica sol with high concentration, high stability and low viscosity, the particle size of silica particles should be increased first.
The advantage of ion exchange method is that silica sol with different properties can be synthesized according to different process combinations, but the disadvantage is that the concentration of sodium silicate as the starting material can not be very high, which leads to a long concentration process and high energy consumption, and a large amount of wastewater generated in the regeneration process of ion exchange resin needs to be treated.
1.2 electrolytic electrodialysis method
Silica sol prepared by this method is an electrochemical method. Its principle is that sodium silicate undergoes hydrolysis reaction in aqueous solution:
na 2 H2 SiO 4+H2O→2Na ++ h3sio 4 –+ OH–
With the progress of the reaction, the ions in the tank will migrate directionally under the action of the electric field, and the impurity ions will be filtered out by the ion exchange membrane; When the concentration of silicic acid generated in the anode chamber is greater than its solubility, polycondensation will occur and silica sol will be generated. The corresponding silica sol can be obtained by adjusting the pH value in the tank. When preparing silica sol by this method, we should pay attention to controlling the current density, temperature and other reaction conditions of electrodialysis reaction.
OKETA YUTAKA of Japan mentioned in its patent that desalted acidic silica sol was prepared by electrodialysis with ion exchange membrane. In the preparation process, desalination chambers and concentration chambers are alternately formed in the electrodialyzer; The anode and cathode are separated by anion exchange membrane, and then electrodialysis is carried out. The temperature of the aqueous solution in the desalting chamber is kept at 5 ~ 20℃.
Electrolytic electrodialysis method is to neutralize sodium silicate aqueous solution with acid, and dialyze sodium ions through semi-permeable membrane after aging. The disadvantage of this method is that the dialysis time is too long and it is not suitable for industrial production.
1.3. Dispersion method
This method is a physical method to prepare silica sol by mechanically dispersing silica particles in water. The specific steps are as follows: weighing a certain amount of deionized water, adding it into a plastic cup and fixing it on a high-speed disperser. Start the high-speed disperser and continuously add a certain amount of fumed silica powder into the cup. After adding SiO _ 2 powder, a certain amount of deionized water was added to adjust the high-speed dispersion speed, and after a certain time, SiO _ 2 aqueous dispersion was prepared. After aging the aqueous dispersion of silica overnight, it was dispersed at high speed and added with additives, and continued to be dispersed at high speed for several hours, and filtered with a 300-mesh screen to obtain silica sol with good performance.
Fu Chaochun acidic silica sol prepared by this method can effectively replace microorganisms in the treatment of human and animal feces and garbage, and can remove odor and prepare efficient organic fertilizer. The specific process is as follows: sulfuric acid with a certain concentration and dispersant SiO2 _ 2 below 200 mesh are put into a plastic container and stirred; Adjusting the pH value to 2-4 with sodium hydroxide; The metal plate is used as an electrode, connected with a rectifying power supply, and put into a plastic container to be electrified; Apply voltage 100 V, electrify for 450 mA for 2 ~ 5min；; After cutting off the power supply of the rectifier, stir for a period of time, and stop stirring when the reactants are colloidal. The content of silica in acidic silica sol prepared by this method is 25% ~ 35%, and the particle size is 1 ~ 12 nm.
Because the acidic silica sol prepared by this method has special uses, the influence of impurity ions such as Na+ and SO42- on its purity is not considered, so this method is not generally suitable for the preparation of acidic silica sol.
1.4. thermal oxidation method of monocrystalline silicon
The research shows that the growth of thermal oxide of silicon is usually carried out in a quartz tube between 900 ~ 1200℃, or under the condition of dry oxygen, or under the condition of wet oxygen containing water vapor, or in the steam formed by passing dry oxygen and nitrogen through nearly boiling water. According to the data, the oxidation rate of elemental silicon in wet oxygen or steam atmosphere is faster than that in dry oxygen. The total reaction of thermal oxidation is:
Silicon+oxygen (gas) → silicon dioxide+2H2O (gas) → silicon dioxide+2H2 (gas)
In the process of dry oxidation, the first reaction dominates, while in the process of wet oxidation, the second reaction dominates.
2. Study on the micelle structure and stability of acidic silica sol.
China started the research and production of silica sol as early as 1958. For example, Institute of Coordination Chemistry of Nanjing University, Institute of Chemical Engineering of Lanzhou Chemical Company, Qingdao Ocean Chemical Plant, etc. Are engaged in related research and development, but the variety and output are far from those of foreign countries, especially the unreasonable ratio of acid-base silica sol. This situation did not improve until the 1980s. Acidic silica sol is in metastable state, and it will gel gradually during the placing process. The stable period is generally 3 ~ 6 months, which is shorter than that of alkaline silica sol. Therefore, how to improve the stability of acidic silica sol has become a concern of many researchers.
2. 1. Colloidal structure of acidic silica sol
Acidic silica sol, also known as silicic acid hydrosol, is a colloidal solution with high molecular weight SiO2 _ 2 particles dispersed in water. It is odorless and nontoxic, and its molecular formula can be expressed as MSIO _ 2·NH2O (where: m, n is very large, and m
The micelle structure is shown in figure 1: when A+ is a metal ion such as n A+, it is expressed as an alkaline silica sol; When A+ is H+, it means acidic silica sol. In the process of movement, the colloidal particles composed of colloidal core and adsorption layer move integrally, so that the diffusion layer forms a dynamic balance with the surrounding electrolyte to maintain the stability of silica sol.
2.2 Factors affecting the stability of acidic silica sol
2.2. Effect of pH value of1.on the stability of acidic silica sol
The close relationship between the stability of silica sol and pH value is shown in Figure 2. As can be seen from fig. 2, at low pH (
Wang et al. think that pH value is directly related to the stability of silica sol. When the pH value of silica sol is between 2 ~ 10, the zeta potential of particles is negative. When the pH value is lower than 2, the zeta potential of the particles is positive. "0" potential at pH=2; The pH value is in the range of 8.5 ~ 10, which is a stable region. When pH> 10, silica sol particles dissolve into silicate; When pH is lower than 4, it is a metastable region. When pH=2, it is the highest metastable state. According to the characteristics of the prepared high-purity silica sol, adjusting the pH value of the silica sol to about 2.5 can keep the sol in a high metastable state and can be stored at room temperature for 2 years without gelation. One of the main manifestations of instability of silica sol is gelation.
Jia Guangyao and others mentioned that the sol-gel kinetics can be controlled artificially. They found that the viscosity, zeta potential and gelation process of silica sol are closely related to pH value, and the gelation process occurs between pH value 4 and 7.
2.2.2 Effect of electrolyte on stability of acidic silica sol
Electrolyte also has a certain influence on the stability of silica sol, which is closely related to pH value. Because the ions released by salts combine with the surface charges of silica sol, the counter ions entering the dense layer increase, which makes the dispersion layer thinner. When the electrolyte concentration increases to a certain extent, the thickness of the dispersion layer is zero, which causes the aggregation and gelation of particles. The degree of gelation is related to the type, concentration and temperature of electrolyte used. According to reports, at pH
J.L. Trompette et al. proposed that concentrated silica sol was easy to gel at pH 9.8 when there were two different compensation ions, and the gel kinetics was studied. The results show that ionic characteristics have an important influence on polymerization kinetics and gel microstructure during sol-gel transformation. This is because the critical condensation concentration is different under the influence of different electrolytes.
Xu Nianqiang and others believe that the stability of silica sol is affected by the concentration of electrolyte salt only when the particle size of SiO _ 2 is small, and the influence of electrolyte salt concentration on the stability of silica sol is weakened with the increase of SiO _ 2 particle size. When the salt content in silica sol is reduced to a certain value, the electrolyte salt concentration will not become the main influencing factor for preparing acidic silica sol to a certain extent.
Yang Jing et al. studied the influence of catalyst type, reaction temperature, reaction time and additives on the properties of silica sol, and analyzed the influence of electrolyte type. Under the same condition of [H+], the influence of acid catalyst on the viscosity of silica sol is as follows:
HF & gtHCl & gt nitric acid & gtH2SO4 & gtHAc, the influence on gel time is: HAc >；; H2SO4 & gtHCl & gt nitric acid & gtHF, the solid content of several sols is: H2SO4 >; Nitric acid & gtHCl & gtHAc and silica sol are suitable for preparing silicon dioxide films with hydrochloric acid or nitric acid as catalysts.
2.2.3. Effect of particle size on acidic silica sol
Particle size is another important factor affecting the stability of silica sol. When the diameter of silica sol particles is within a certain range, the more uniform the particle size, the narrower the distribution range and the better the stability.
When studying the influence of particle size on acidic silica sol, Xu Nianqiang and others mentioned that at a certain concentration, the relationship between the stability of acidic silica sol and the particle size of SiO2 _ 2 showed an oblique "S" shape, that is, at a small particle size, the stability of silica sol was relatively low, but with the increase of particle size, the stability of silica sol increased rapidly. The particle size was in the range of 10 ~ 20 nm, and the stability of silica sol was approximately proportional to the particle size.
Some scholars have found that controlling the particle size of silica sol in the range of 10 ~ 15 nm can not only simplify the process, but also maintain the stability of high-purity silica sol.
In addition, with the increase of the radius of silica particles, the reactivity of hydroxyl groups on the surface of silica particles will decrease, the specific surface area of colloidal particles will decrease, and the adsorption energy of colloidal particles will decrease, thus the adsorption force of large particles on small particles will decrease, which is also the reason why the stability of large particle acidic silica sol is higher than that of small particle silica sol.
In addition, when preparing silica sol, Janne Puputti and others replaced part of water with ethanol, which improved its stability by three times. Anna Schantz Zackrisson and others studied the polymerization and gelation process in silica sol dispersion system by interference method and time-resolved small angle X-ray scattering, and analyzed the influence of ionic strength on the critical point of gel.